The Ijen volcano complex is a group of stratovolcanoes in the Bondowoso Regency of East Java, Indonesia. It is inside a larger caldera Ijen, which is about 20 kilometers wide. The Gunung Merapi stratovolcano is the highest point of that complex. The name “Gunung Merapi” means “mountain of fire” in the Indonesian language (api being “fire”); Mount Merapi in central Java and Marapi in Sumatra have the same etymology.

“This blue glow—unusual for a volcano—isn’t, of course, lava, as unfortunately can be read on many websites,” Grunewald told National Geographic in an email about Kawah Ijen, a volcano on the island of Java.

The glow is actually the light from the combustion of sulfuric gases, Grunewald explained.

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